End-burning grains are used in solid propellant rocket motors and pyrotechnic gas generators that require stable and prolonged generation of combustion gases. In addition to these qualities, end-burning grains reduce the chances of spalling of the propellant and of unintentional ignition upon impact from bullets or fragments. An end-burning grain also allows a motor to accommodate a high volume of propellant, and when desired, the propellant can be loaded in a cartridge for easy placement in the motor case. Examples of end-burning rocket motors are the original Stinger (a surface-to-air missile developed by the United States Army), the ARCAS (All-Purpose Rocket for Collecting Atmospheric Soundings, first developed by Atlantic Research Corporation), and the Standard Missile Sustainer motors (also developed by Atlantic Research Corporation).
In an end-burning motor, the length of the propellant grain establishes the duration of the thrust, while the magnitude of the thrust depends on the mass burning rate which is determined by the choice of propellant as well as the area of the burning surface. The mass burning rates of solid propellants are limited, however, and while propellant compositions can be modified to achieve higher burn rates, these modifications tend to produce smoke, toxic gases, or both. The smoke emitted from shoulder-launched rockets such as the Stinger obscures the user's vision and risks eye damage, and smoke emitted from tactical rockets launched from aircraft or ground vehicles impairs the visibility of the pilot or driver. Smoke also leaves a visible trail that can indicate the source of the launched rocket.
To address these problems, reduced-smoke and minimum-smoke propellants have been developed. Unfortunately, the ballistic properties of such propellants, particularly the minimum-smoke propellants, are generally undesirable because their burning rates are temperature-dependent, as opposed to “plateau” or “mesa” burning rates, i.e., burning rates that are insensitive to temperature over a wide range of operating pressures. Propellants with “plateau” or “mesa” burning rates are capable of providing substantially constant thrust regardless of firing conditions and operating temperature. Burning rate modifiers that are specifically designed to achieve “plateau” or “mesa” burning rate behavior are often included in minimum-smoke propellants, and the modifiers of choice for this purpose are lead salts and lead-containing compounds, as described in U.S. Pat. No. 3,138,499 (A. T. Camp et al., inventors, issue date Jun. 23, 1964). Lead salts and related compounds present problems, however. They are toxic, they complicate the propellant manufacturing process, they produce exhaust products that are hazardous to personnel, particularly during training exercises involving the use of the rockets, and they are harmful to the environment in which the training exercises are conducted. For these reasons, training in the use of rockets with minimum-smoke propellants must include expensive range remediation from the toxic by-products.
Enlargement of the area of the burning surface to increase thrust can be achieved by increasing the motor diameter, but this too has its limitations, since there are practical limitations on how large the motor diameter can be.